Serotonergic mechanisms in anxiety.

1. Anatomical, behavioral, neurochemical and electrophysiological evidence collectively support a role for central 5-HT in the modulation of anxiety and the anti-anxiety action of the benzodiazepines. 2. The advent of selective agonists and antagonists for 5-HT receptor subtypes (5-HT1, 5-HT2, 5-HT3) has rekindled investigation of the role of 5-HT in anxiety mechanisms. 3. The azapirones represent a new class of agent which possesses affinity for 5-HT1A receptors (partial agonists) and is active in anxiolytic animal models as well as in the clinic (buspirone) 4. While preclinical data supporting the anxiolytic potential of 5-HT2 antagonists remains controversial, a recent clinical study supports ritanserin's anxiolytic effects. 5. Several animal models support the anxiolytic potential of the 5-HT3 antagonist odansetron (GR38032F). Confirmation of it's therapeutic utility awaits clinical results.

The nootropic compound BMY-21502 improves spatial learning ability in brain injured rats.

Although long-lasting cognitive dysfunction often follows clinical traumatic brain injury (TBI), few pharmacologic regimens have been developed to treat post-traumatic cognitive deficits. We have previously shown that, in the rat, experimental lateral fluid-percussion (FP) brain injury induces a profound impairment in retrograde memory. In the present study, we characterized alterations in the ability of rats to learn a novel task following lateral FP brain injury and examined the potential modulatory effects of the nootropic cognitive enhancer BMY-21502 on post-injury learning. Male Sprague-Dawley rats were subjected to lateral (parasagittal) FP brain injury of moderate severity (2.4 atm) or sham surgery (no injury). On days 7 and 8 post-injury, animals were tested in a Morris water maze for their ability to learn to navigate to a submerged, invisible platform using external visual cues. BMY-21502 (10 mg/kg) or vehicle was administered 30 min prior to the first trial on both days. A highly significant (P < 0.001) impairment in post-injury learning was observed in vehicle-treated brain-injured animals compared with vehicle-treated sham animals. Injured animals treated with BMY-21502 at one week post-injury showed significantly (P < 0.05) improvement in post-injury learning ability compared to injured animals treated with vehicle. Paradoxically, in uninjured control animals BMY-21502 treatment appeared to worsen learning scores. The results of this study indicate that BMY-21502 may be useful for attenuating the dysfunction in learning ability that occurs following TBI.

A role for sigma binding in the antipsychotic profile of BMY 14802?

BMY 14802 was identified as a potential antipsychotic drug in traditional model systems, and this identification was confirmed in modern behavioral and electrophysiological systems. The drug appears to be atypical as an antipsychotic in its lack of activity in models predictive of the potential to produce extrapyramidal side effects and tardive dyskinesia. Indeed, this suggestion is corroborated by clinical findings to date. The atypical profile of BMY 14802 extends to its neurochemical actions and appears to find its basis in regionally selective, indirect modulation of the dopamine system. Furthermore, BMY 14802 exhibits interactions with sigma binding sites in vitro and in vivo, a notion supported by data from neurophysiological, behavioral, and biochemical investigations. BMY 14802 also appears to be neuroprotective in some model systems and may have utility in the treatment of stroke (Boissard et al. 1991). BMY 14802 appears to interact with 5-HT1A receptors, but this interaction does not seem to contribute significantly to the potential antipsychotic actions of the drug. Moreover, the formation of active metabolites of BMY 14802 does not appear to occur in animals or humans to an extent of physiological or behavioral relevance. If clinically efficacious, BMY 14802 may treat the symptoms of schizophrenia by a mechanism novel for antipsychotic drugs: regionally selective, indirect modulation of dopaminergic systems by specific interaction at sigma sites.

Synthesis and biological characterization of alpha-(4-fluorophenyl)-4-(5-fluoro-2-pyrimidinyl)-1-piperazinebutanol and analogues as potential atypical antipsychotic agents.

A series of 1-(pyrimidin-2-yl)piperazine derivatives were prepared and evaluated in receptor binding assays and in in vivo behavioral paradigms as potential atypical antipsychotic agents. Compound 16 (BMS 181100 (formerly BMY 14802)) emerged as the lead compound from within the series on the basis of its good activity and duration of action in the inhibition of both conditioned avoidance responding and apomorphine-induced stereotopy in the rat. Compound 16 not only failed to induce catalepsy in the rat but was quite effective in reversing the cataleptic effect of neuroleptic agents, thus indicating a low propensity for causing extrapyramidal side effects. In comparison to reference antipsychotic agents, 16 appeared to be less sedating and was relatively weaker in causing muscle incoordination. The compound was essentially inactive in binding to dopamine D2 receptors and its chronic administration to rats did not result in dopamine receptor supersensitivity. It exhibited modest to weak affinity for 5-HT1A and alpha 1 receptors but was found to be a fairly potent ligand for sigma binding sites (IC50 vs (+)-[3H]-3-PPP = 112 nM). Although the resolved enantiomers of racemic 16 did not show dramatic differences from racemate or from each other in most tests, the R(+) enantiomer was up to 11-fold more potent than its antipode in binding to sigma sites. Several studies have indicated that 16 may be a limbic-selective agent which may modulate dopaminergic activity by an indirect mechanism. The compound has been selected for clinical evaluation in the treatment of psychosis.

Potentiation of opioid analgesia by the antidepressant nefazodone.

Nefazodone is a new antidepressant related structurally to trazodone. In addition to its activity in preclinical assays for antidepressant activity, nefazodone was a potent analgesic in the mouse hotplate assay. At 50 mg/kg s.c. nefazodone doubled baseline latencies in 40% of mice but was inactive in the tailflick test at any dose tested. The hotplate analgesia seen with nefazodone alone was not reversed by naloxone (10 mg/kg s.c.). In the tailflick assay, nefazodone (50 mg/kg s.c.) enhanced morphine's analgesic response, shifting morphine's ED50 from 3.1 mg/kg alone to 0.86 mg/kg in conjunction with nefazodone (P less than 0.05). Two days after implantation of a morphine pellet (75 mg) no mice remained analgesic in the tailflick assay. Administration of nefazodone (50 mg/kg s.c.) restored analgesia to 60% of mice (P less than 0.03). In selective analgesic assays, nefazodone enhanced mu 1, mu 2 and delta analgesia, but not kappa 1 or kappa 3 analgesia. Nefazodone did not affect morphine's LD50 and, in assays of gastrointestinal transit, nefazodone increased morphine's potency only slightly. In conclusion, nefazodone alone is analgesic in certain animal models. In conjunction with morphine, nefazodone potentiated analgesia with no effect on lethality and little effect on gastrointestinal transit, resulting in an increase in morphine's therapeutic index. These results suggest that nefazodone and similar agents may have a significant role in the management of pain.

Serotonin: a common neurobiologic substrate in anxiety and depression.

Anxiety and depression have traditionally been considered independent pathologies. Recent evidence suggests that these disorders may be related. Some tricyclic antidepressant drugs appear to be effective against generalized anxiety disorders in man, and in animal models, chronic administration of some anxiolytic drugs produces activities predictive of antidepressant potential. It is proposed that imbalances in serotonergic neurotransmission may contribute significantly to both pathologies and that agents that act as serotonin partial agonists may normalize neurotransmission in both serotonin deficit (depression) and excess (anxiety) diseases. The azapirones, a new class of drugs that includes buspirone, ipsapirone, and gepirone, act as serotonin partial agonists at the serotonergic type1A receptor, and may prove useful in treating multiple psychopathologies characterized by aberrant serotonin neurotransmission.

Nefazodone: preclinical pharmacology of a new antidepressant.

Recent pharmacologic studies suggest that nefazodone may possess antidepressant activity. Nefazodone is active in behavioral models predictive of antidepressant potential. It is active in reversing learned helplessness, prevents reserpine-induced ptosis, and enhances response efficiency in the differential reinforcement for low rates of response paradigm. In in vitro studies, nefazodone inhibits the binding of [3H]ketanserin to cortical serotonin2 (5-HT2) binding sites, whereas in vivo, it antagonizes the 5-HT2-mediated quipazine-induced head shake in rats. In ex vivo studies, acute oral administration of nefazodone inhibits cortical serotonin uptake and occupies frontal cortical 5-HT2 receptor binding sites. Chronic administration of nefazodone produces a reduction in 5-HT2-mediated behavior and decreases cortical 5-HT2 receptor binding site density. Further, a chronic high-dose nefazodone regimen significantly potentiates 5-HT1A-mediated behavioral responses in rats. Nefazodone exhibits decreased anticholinergic, alpha-adrenolytic, and sedative activity relative to other antidepressants.

The new generation of serotonergic anxiolytics: possible clinical roles.

Serotonin has been implicated in mediating diverse physiologic and psychologic processes. The anatomy and complex pharmacology of brain-serotonin systems enables this neurotransmitter to broadly affect normal and abnormal behaviors. It appears that serotonin plays a role in multiple psychopathologies, including anxiety, depression, mood disorders, aggressive acting out, alcohol-related syndromes, and disinhibitory disorders characterized by impulsivity. It would not be surprising, therefore, if drugs that alter the dynamics of serotonergic neurotransmission prove to be effective in multiple clinical settings. Such agents may treat broad symptom clusters common to multiple nosologic categories. The new generation of serotonergic anxiolytics, including buspirone, gepirone, ipsapirone, and SM-3997, which interact potently with 5-hydroxytryptamine-1A receptors, may prove to be such symptom cluster drugs. There is a scientific rationale for exploring the clinical utility of these agents in anxiety, depression, mood disorders, aggressive syndromes, and alcohol-related disorders.

Effects of imipramine and serotonin-2 agonists and antagonists on serotonin-2 and beta-adrenergic receptors following noradrenergic or serotonergic denervation.

The effects of chronic (14 day) administration of the tricyclic antidepressant imipramine, the serotonin-2 (5-HT2) antagonist ketanserin, and the serotonin agonist quipazine on 5-HT2 receptor binding parameters and 5-HT2-mediated behavior were examined in rats with or without prior serotonergic denervation [via 5,7-dihydroxytryptamine (5,7-DHT)] or noradrenergic denervation [via N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4)]. Chronic administration of imipramine, ketanserin, or quipazine produced a marked reduction in the number of 5-HT2 binding sites which was accompanied by reductions in the 5-HT2-mediated quipazine-induced head shake response. In animals receiving DSP4 or 5,7-DHT lesions and continuous vehicle treatment, beta-adrenergic receptor binding sites were significantly up-regulated while 5-HT2 receptor binding sites did not change. Imipramine normalized the lesion-induced increases in beta-adrenergic binding observed in DSP4 and 5,7-DHT-lesioned rats but failed to down-regulate beta-adrenergic binding sites below non-lesioned control levels. Chronic imipramine, ketanserin, and quipazine reduced quipazine-induced head shakes and down-regulated 5-HT2 binding sites in rats with noradrenergic denervation. While imipramine, ketanserin, and quipazine all down-regulated 5-HT2 binding sites in animals with serotonergic denervation, only imipramine's ability to reduce quipazine-induced head shakes was attenuated in 5,7-DHT-lesioned rats. The present results suggest that imipramine-induced down-regulation of 5-HT2 receptors may not involve presynaptic 5-HT mechanisms, and imipramine-induced alterations in 5-HT2 sensitivity as reflected in the quipazine-induced head shake may, in part, be influenced by beta-adrenergic receptors.

Atypical antipsychotic agents: patterns of activity in a series of 3-substituted 2-pyridinyl-1-piperazine derivatives.

A series of 3-substituted 2-pyridinyl-1-piperazine derivatives have been appended to cyclic imide groups and evaluated for their potential antipsychotic activity. The dopamine receptor affinities of these target molecules, as well as their ability to block apomorphine-induced stereotypy or reverse neuroleptic-induced catalepsy, was dependent on the lipophilic and electronic characteristics of the substituent situated on the pyridine ring. Groups with + omega and - phi values were most consistent with the desired biological profile of the target molecules, the cyano moiety being the optimum choice. Evaluation of compound 12 in a monkey model of amphetamine psychosis, and the regional selectivity it expresses for the A10 dopaminergic cell bodies in electrophysiological experiments, suggest this compound would be an atypical antipsychotic agent with few side effects.

Buspirone analogues. 2. Structure-activity relationships of aromatic imide derivatives.

Several analogues of the novel anxiolytic buspirone were synthesized and evaluated in vivo for tranquilizing activity and their ability to reverse neuroleptic-induced catalepsy. The in vitro binding affinities of these compounds were also examined for both the alpha 1 and dopamine D2 receptor systems. The general structure-activity relationships of this series highlight compounds 17, 21, and 32 as having anticonflict activity. Each of these structures contains the 1-(2-pyrimidinyl)piperazine moiety linked by a tetramethylene chain to a variable cyclic imide moiety. Compound 32 (4,4-dimethyl-1-[4-[4-(2-pyrimidinyl)-1-piperazinyl]butyl]-2,6- piperidinedione) was found to be equipotent with buspirone in its anxiolytic activity and was therefore selected for extensive preclinical characterization. The pharmacology of buspirone and 32 is contrasted, and the potent serotonin agonist properties of 32 are discussed with reference to its potential contribution to the anxioselective mechanism of this compound.

Synthesis and biological evaluation of 1-(1,2-benzisothiazol-3-yl)- and (1,2-benzisoxazol-3-yl)piperazine derivatives as potential antipsychotic agents.

Members of the series of title compounds were tested for potential antipsychotic activity in relevant receptor binding assays and behavioral screens. Structure-activity relationships within the series are discussed. Compound 24 (BMY 13859-1), a (1,2-benzisothiazol-3-yl)piperazine derivative, was selected for further study because of its potent and selective profile in primary CNS tests. It was active in the Sidman avoidance paradigm and blocked amphetamine-induced stereotyped behavior in dogs for up to 7 h. The compound's lack of typical neuroleptic-like effects in the rat catalepsy test and its failure to produce dopamine receptor supersensitivity following chronic administration indicate that it should not cause the movement disorders commonly associated with antipsychotic therapy. Although 24 has potent affinity for dopaminergic binding sites, its even greater affinity for serotonin receptors suggests that a serotonergic component may be relevant to its atypical profile. Compound 24 is currently undergoing clinical evaluation in schizophrenic patients.

Serotonergic mechanisms in the behavioral effects of buspirone and gepirone.

The literature describing the role of serotonin (5-HT) in the mediation of anxiety is a controversial one. Serotonergic involvement in the mechanism of action of two nonbenzodiazepine anxiolytics, buspirone and gepirone, supports a role for serotonin in anxiety. The anticonflict effect of both drugs is blocked by serotonin lesions, and gepirone induces the serotonin syndrome. A shift in the gepirone dose-response curve to the left in serotonin lesioned rats suggests that this may be 5-HT-receptor mediated. Both buspirone and gepirone enhance the acoustic startle response and gepirone's effect is attenuated in serotonin lesioned animals. While other components of buspirone's mechanism of action may suppress the behavioral expression of its serotonergic interactions, results from these studies suggest that serotonin agonist-like activity may be an important mechanism in the actions of a clinically proven nonbenzodiazepine anxiolytic (buspirone), and anxiolytic candidate (gepirone).

Lack of withdrawal signs of dependence following cessation of treatment or Ro-15,1788 administration to rats chronically treated with buspirone.

Failure to gain weight during withdrawal from chronic regimens of dependence-producing drugs is observed in laboratory rats. Withdrawal from the benzodiazepine anxiolytic agent diazepam, whether induced by cessation of treatment or precipitated by administration of the specific benzodiazepine antagonist Ro-15,1788, is accompanied by weight loss. However, the same withdrawal treatments following a chronic regimen of equal doses of the nonbenzodiazepine antianxiety drug buspirone are not associated with weight loss. Buspirone-treated rats gain weight during withdrawal, suggesting that this novel anxiolytic should lack dependence liability in man.

Pharmacological and clinical effects of buspirone.

Clinical trials have demonstrated that buspirone (BuSpar) is effective in the treatment of anxiety with efficacy and dosage comparable to diazepam or chlorazepate. Buspirone has a unique structure and a pharmacologic profile which distinguishes it from the benzodiazepines. Because it lacks the anticonvulsant, sedative, and muscle-relaxant properties associated with other anxiolytics, buspirone has been termed "anxioselective." Animal studies suggest that it lacks potential for abuse, and this finding is supported by clinical investigations. Further preclinical work supports the contention that buspirone lacks liability to produce physical dependence or to significantly interact with central nervous system depressants such as ethanol. Moreover, biochemical investigations have not identified any direct interaction of buspirone with the benzodiazepine-gamma-aminobutyric acid-chloride ionophore complex. Pharmacologic studies on the molecular level indicate that buspirone interacts with dopamine and serotonin receptors. Recent behavioral, electrophysiological, and biochemical studies have clearly demonstrated that early hypotheses that buspirone might be considered a neuroleptic are no longer tenable. Recent evidence indicates that other neurotransmitter systems (serotonin, norepinephrine, acetylcholine) mediate buspirone's effects. It is hoped that future studies can define the mechanism by which buspirone alleviates the clinical manifestations of anxiety.

The effects of buspirone, BMY-13805 and 1-(2-pyrimidinyl)-piperazine on cat spinal reflexes.

Buspirone and its analog BMY-13805, clinically effective anxiolytics and their metabolite, 1-PP, were tested on cat spinal reflexes. In the spinal preparation, the monosynaptic reflex and the dorsal root potential were not changed by buspirone, BMY-13805 or 1-PP. Significant changes in the dorsal root potential and monosynaptic reflex were seen after buspirone administration in the intact neuraxis preparation. Generally, these changes had a bimodal pattern, a significant change within 1 hr after buspirone administration, then a period of remittance, followed by a second significant change 3 or more hr later. They included increases in the dorsal root potential and the following changes in the conditioned monosynaptic reflex: an increase in excitatory postsynaptic potential, an increase in postsynaptic inhibition, and decreases in presynaptic inhibition. These data indicate that buspirone does not affect these reflexes directly, but alters supraspinal mechanisms that regulate spinal reflexes.

Determination of functionally-relevant serum levels of MJ 13859-1 in the dog: relationship to blockade of amphetamine stereotypy.

The ability of drugs to block dopamine-agonist induced stereotypy in animals has proven to be a reliable in vivo predictor of antipsychotic efficacy in man. Assessment of a drug's potency at displacing [3H]spiperone from rat striatal membranes is an in vitro test which also has predictive validity for antipsychotic actions. Methods are described for assessing stereotyped behavior in the dog, and for measuring the ability of serum samples taken from treated animals at behaviorally interesting time points, to displace [3H]spiperone from washed synaptic membranes of rat striata. In these studies, oral administration of the potential antipsychotic agent MJ 13859-1 blocked amphetamine-induced stereotyped behavior in the dog for 6 hours. This behavioral effect was accompanied by sustained serum levels of [3H]spiperone displacing moieties derived from MJ 13859-1.

Use of animal models: toward anxioselective drugs.

The debilitating psychopathology of anxiety neurosis is particularly amenable to relief by pharmacotherapy. While currently available drugs successfully alleviate the distressing symptoms suffered by anxious patients, they also carry potential liabilities which must be considered in their use. Through the use of predictive animal models, researchers hope to elucidate a set of structure-activity relationships through which anxiolytic, and most particularly anxioselective compounds can be rationally designed. Such models should predict side-effect potential as well as efficacy, so that unwanted ancillary effects of potential anxiolytics can be eliminated by appropriate structural modifications. The state of the art in preclinical testing for anxiolytic potential will be discussed with particular emphasis on the need to design test systems capable of detecting anxiolytic activity in diverse, nontraditional chemical series. The contribution of such methodology to the discovery and development of one anxioselective, non-benzodiazepine anxiolytic, buspirone, will be discussed.